Wolsong Unit 1, a domestic heavy water reactor nuclear power plant, was permanently shut down in December 2019. Accordingly, Wolsong Unit 1 plans to prepare a Final Decommissioning Plan (FDP), submit it to the government by 2024, receive approval for decommissioning, and begin full-scale decommissioning. One of the important tasks in the decommissioning of Wolsong Unit 1 is to determine the decommissioning strategy. It is necessary to decide on a decommissioning strategy considering various factors and variables, secure the technical background, and justify it. The selection of a decommissioning strategy is best achieved through the use of formal decisionmaking assistance techniques, such as considerations related to influencing factors. It is very important to understand the basic decommissioning strategy alternatives and whether sufficient consideration has been given to situations where only a single unit is permanently shut down in a multi-unit site like Wolsong Unit 1, while the remaining units are in normal operation. As a process for selecting a decommissioning strategy, first, all considerations that could potentially affect decommissioning presented in the KINS Decommissioning Safety Review Guidelines were synthesized, influencing factors to be used in the decision-making process were determined, and the concept was defined. In order to select the most appropriate decommissioning strategy by considering various evaluation attributes of possible decommissioning alternatives (immediate dismantling and delayed dismantling), the Wolsong Unit 1 decommissioning strategy was evaluated by reflecting the AHP decision-making technique.

KHNP is conducting research to decommission Wolsong Unit 1 Calandria. Establishment of preparation and dismantlement processes, conceptual design of equipment and temporary radiation protection facilities, and waste management are being established. In particular, the ALARA plan is to be established by performing exposure dose evaluation for workers. This study aims to deal with the methodology of evaluating exposure dose based on the calandria dismantling process. The preparation process consists of bringing in and installing tooling and devices, and removing interference facilities to secure work space. The main source term for the preparation process is the calandria structure itself and crud of feeders. In the case of the dismantlement process, a structure with a shape that changes according to the process was modeled as a radiation source. It is intended to estimate the exposure dose by selecting the number of workers, time, and location required for each process in the radiation field evaluated according to the preparation and dismantlement process. In addition, it is also conducting an evaluation of the impact on dust generated by cutting operations and the human impact of C-14, H-3, which are specialized nuclides for heavy water reactors. KHNP is conducting an exposure dose evaluation based on a process based on the preparation and dismantlement process for decommissioning Calandria through computation code analysis. If additional worker protection measures are deemed necessary through dose evaluation according to this methodology, the process is improved to prepare for the dismantling of worker safety priorities.

Wolsong Unit 1 is about 679 MW Pressurized Heavy Water Reactor (PHWR). Canada AECL was responsible for Nuclear Steam Supply System (NSSS) design and supply. Wolsong Unit 1 was operated from 1983 to 2019. Currently, Wolsong Unit 1 is under safety management after permanent shutdown. Wolsong unit 1, a heavy water reactor, has the following characteristics. • Unlike Light Water Reactor, vertical reactors, Heavy Water Reactor is installed horizontally. • The internal structure of the reactor is more complex than that of a light water reactor (380 pressure tubes in reactor as called Calandria) • The Calandria Vault, a large concrete structure filled with light water, is located outside of Calandria In the case of the decommissioning plan of PHWR in Canada, they have adopted a deferred decommissioning strategy that decommissioning begins after permanent shutdown and long-term safety management (30 to 40 years). So, Decommissioning of PHWR in Canada is expected to start in the 2050s. Nuclear Safety Act stipulates that if a commercial nuclear power plant is permanently suspended, the utility must submit a Final Decommissioning Plan (FDP) within 5 years. So, KHNP, the utility, is developing the FDP for Wolsong Unit 1 and have a plan to submit it to the government by the end of 2024. And then licensing review is expected to take at least two years. The key milestone for decommissioning project has a plan to start decommissioning in 2027 and complete it by 2034, but this is flexible depending of the government’s approval for decommissioning and the completion of prerequisites such as spent fuel transfer, etc. KHNP has prepared a strategy and system consisting of three areas such as R&D, Engineering and licensing document development to prepare the final decommissioning plan for Wolsong Unit 1. The promotion system for the preparation of the FDP for Wolsong Unit 1 is consisted of Engineering (HAS Characterization, Process/Work Package/Cost Estimation, Dismantling Safety Evaluation, Radiological Environmental Report, Radioactive Waste Treatment and Facility Construction), R&D (COG cooperation, KHNP R&D Results), Kori unit 1 lessons learned, etc. KHNP have the plan that the FDP Draft development by the end of 2023, reflecting engineering services results, R&D results, COG technical cooperation results and lessoned learned on Kori Unit 1. After collecting opinions from residents through a public hearing, the FDP will be submitted to the government by the end of 2024. It is expected that there will be many difficulties in the development process as it is the world’s first FDP development for the commercial Pressurized Heavy Water Reactors.